FCC-LPG SHU

Purpose
Diolefins and mercaptans are contaminants to all alkylation and dimerization processes. Eliminating acetylenes and diolefins from an olefins-rich stream can produce a number of potential benefits, such as a reduction in:

• acid consumption in alkylation;
• catalyst fouling in dimerization;
• product gum concentration; and
• energy required for redistillation of the product.

Description
An olefin rich hydrocarbon from debutanizer enters the feed drum. The feed is then pumped to 300 – 450 psig, mixed with hydrogen, heated and then introduced to the reactor. The hydrogen flow is regulated in proportion to the hydrocarbon feed rate.

The purified product exits the reactor, is cooled and enters the product stripper. Un-reacted hydrogen and light hydrocarbons are vented back to the wet gas compressor. The stabilized product is cooled prior being routed to the LPG Splitter.

Catalyst Performance Metrics
The most important function of the catalyst is conversion of acetylene and diolefins. High conversion rates require a combination of activity and selectivity.

ACTIVITY is gauged by the consumption of hydrogen at a given LHSV (liquid hourly space velocity — the inverse of residence time) and temperature. Ideal activity achieves high consumption at high LHSV at low temperature. Certain contaminants in the feed can adversely affect this outcome. A catalyst must be designed to overcome the inhibiting effects of the contaminants. Catalyst activity also determines how much unreacted hydrogen remains in the reactor effluent.

SELECTIVITY compares the relative amount of olefin saturated to the amount of acetylenes and diolefins converted. Because selectivity is a fundamental characteristic of the catalyst, selectivity becomes the focus of much design work. Without adjusting selectivity, all the hydrogen could be consumed before all the acetylenes and diolefins react. Selectivity also determines factors such as: the amount of hydrogen required to meet the product specification; how much olefin is converted to parafin while achieving the required acetylene and diolefin specification; and the temperature rise in the reactor.

An additional function of this process is hydroisomerization of 1-butene to form
trans-2- butene. This is achieved by manipulation of process conditions. However, the catalyst design can also be used to alter the relative rates of hydroisomerization versus olefin saturation.